In recent years, from the viewpoint of global environment protection, there has been a demand for weight reduction in vehicle bodies. High strength steel sheets are widely employed these days for vehicle bodies, in particular, for peripheral components to a passenger compartment (cabin), which contributes to reduction in weight of the vehicle body by thinning the walls thereof. On the other hand, the strength of high strength steel sheets used for an engine room and frames (including a front frame and a rear frame) of a trunk for the purpose of increasing strength merely reaches 780 MPa at maximum. The reason is that the high strength steel sheet for use as a material for a front frame and a rear frame cannot be increased excessively because it involves the following problems and does not necessarily lead to that much of an increase in impact energy absorption amount compared to the increase in strength. That is, the front frame or the rear frame, which serves as a collision energy absorbing member which undergoes significant deformation upon collision to absorb energy of the collision, may be deteriorated in ductility to suffer significant fracture, or has an unstable deformed shape upon collision failing to attain stable buckling, with the result that local fractures may easily occur, when the material steel is increased in strength.
Under the aforementioned circumstances, there is a demand for a collision energy absorbing member having a property of efficiently absorbing energy upon collision while increased in strength for the purpose of promoting the strength of the collision energy absorbing member forming a front frame or a rear frame and of attaining further weight reduction in a vehicle body.
To meet such demand, for example, JP 2001-130444 A discloses a collision energy absorbing member formed of a steel sheet having a microstructure including austenite in an area ratio of 60% or above. JP 2001-130444 A further discloses, as an example of the steel sheet having a microstructure including austenite in an area ratio of 60% or above, an austenite-based stainless steel sheet containing Cr by 18% to 19% and Ni by 8% to 12%, illustrating that a collision energy absorbing member formed by using the aforementioned steel sheet can be improved in deformation propagation properties upon collision to thereby ensure a desired collision energy absorbing performance.
JP H11-193439 A discloses a high strength steel sheet with good workability and having high dynamic deformation resistance. The high strength steel sheet illustrated in JP H11-193439 A has a multi-phase containing: ferrite and/or bainite, either one of which being used as a main phase; and a tertiary phase containing retained austenite by 3% to 50% in volume fraction, and has high dynamic deformation resistance in which, after a pre-deformation of more than 0% to 10% or less, a difference between a strength under quasi-static deformation σs and a dynamic deformation strength σd (σd−σs) satisfies at least 60 MPa, the strength under quasi-static deformation σs being obtained when the steel sheet is deformed at a strain rate of 5×10−4 to 5×10−3 (1/s), the dynamic deformation strength σd being obtained when the steel sheet is deformed at a strain rate of 5×102 to 5×103 (1/s), and the work-hardening exponent at a strain of 5% to 10% satisfies at least 0.130. According to JP H11-193439 A, a member manufactured by using a steel sheet having (σd−σs) of at least 60 MPa is capable of absorbing higher energy upon collision, as compared to a value estimated from the material steel sheet strength.
Further, JP 2007-321207 A discloses a high strength steel sheet having a multi-phase microstructure formed of a ferrite phase and a hard secondary phase contained in an area ratio of 30% to 70% with respect to the entire microstructure, the ferrite phase and the hard secondary phase being dispersed into the steel sheet, in which the area ratio of ferrite having a crystal grain diameter of 1.2 μM or less in the ferrite phase is 15% to 90%, and a relation between the average grain diameter ds of ferrite having a crystal grain diameter of 1.2 μm or less and an average grain diameter dL of ferrite having a crystal grain diameter exceeding 1.2 μm satisfies dL/ds≧3. The technology disclosed in JP 2007-321207 A is capable of improving the balance between strength and ductility that is important upon press forming, to thereby obtain a high strength steel sheet excellent in energy absorbability upon high speed deformation, so that the high strength steel sheet thus obtained can be applied to a vehicle body which requires high collision energy absorbing performance.
Further, according to JP 2008-214645 A and JP 2008-231541 A, studies were made, using a recess introduced rectangular tubular member, on steel sheets capable of being deformed upon axial collapse deformation without crumbling and cracking, and it was found that the amount and size of ferrite, bainite, austenite, and precipitates may be controlled so as to allow the steel sheet to deform without causing crumbling and cracking in the deformation mode upon collision.
Further, Y. Okitsu and N. Tsuji; Proceedings of the 2nd International Symposium on Steel Science (ISSS 2009), pp. 253-256, Oct. 21-24, 2009, Kyoto, Japan: The Iron and Steel Institute of Japan shows examples of a hat profile parts that stably crushes into a bellows shape upon collision crushing. The member is formed of a thin steel sheet having a tensile strength of 1155 MPa and an ultrafine grain multi-phase microstructure, in which n-value is 0.205 for a true strain in a range of 5% to 10%. The thin steel sheet described in Y. Okitsu and N. Tsuji; Proceedings of the 2nd International Symposium on Steel Science (ISSS 2009), pp. 253-256, Oct. 21-24, 2009, Kyoto, Japan: The Iron and Steel Institute of Japan has a chemical composition based on: 0.15% C-1.4% Si-4.0% Mn-0.05% Nb, and has a microstructure including ferrite and a secondary phase each being in submicron size, the secondary phase containing retained austenite by 12% to 35%, that is high in n-value and in strain hardenability.
According to JP 2001-130444 A, the collision energy absorbing member is formed of a steel sheet containing a large amount of austenite. Austenite has a face centered cubic (fcc) crystal structure, and thus has a feature in that it is less susceptible to embrittlement and fracture, which can increase to a certain degree the amount of energy to be absorbed upon collision. However, the steel sheet containing a large amount of austenite as disclosed in JP 2001-130444 A has a low tensile strength of about 780 MPa, and further the strength thereof is lower as compared to a steel sheet having a body centered cubic (bcc) structure when deformed at a high strain rate such as upon collision, which lacks sufficient strength for use as a material for a vehicle collision energy absorbing member. In addition, the Ni content and the Cr content need to be increased to obtain a steel sheet containing a large amount of austenite, which leads to an increase in manufacturing cost. From this point of view, the steel sheet of JP 2001-130444 A is unsuitable for use in a vehicle body member.
According to the technology of JP H11-193439 A, the hat-type member was only evaluated for a steel sheet having a tensile strength of about 780 MPa at maximum. A member formed of a steel sheet having a tensile strength of less than 980 MPa is easily deformed into a bellows shape upon collision deformation without suffering fracture and breakage, and thus the energy to be absorbed by the member upon collision deformation can be estimated based on the material properties. In contrast, a member formed of a steel sheet having a tensile strength of 980 MPa or above suffers fracture and breakage upon collision deformation, and thus the energy to be absorbed by the member upon collision often shows a value lower than expected from the material properties. The technology of JP H11-193439 A has difficulty in suppressing fracture and breakage upon high-speed crush of the member formed of a high strength steel sheet having a tensile strength of 980 MPa or above to thereby stably improve the energy to be absorbed upon high speed crush.
According to the technology described in JP 2007-321207 A, the steel sheet has a mixed microstructure of nanocrystal grains and microcrystal grains, in which the type and the microstructure fraction of the hard secondary phase are optimized, to thereby obtain a high strength steel sheet that is high in strength while having high ductility. However, JP 2007-321207 A gives no description about forming a collision energy absorbing member using the steel sheet, and makes no reference to suppressing fracture and breakage, which otherwise become problematic when a member is formed of a steel sheet having a tensile strength of 980 MPa or above, in the member upon collision to allow the member to be axially stably buckled into a bellows shape to efficiently absorb collision energy, which thus remains unclear.
Further, according to the technology described in JP 2008-214645 A and JP 2008-231541 A, C, Si, Mn, and Ti and/or Nb are each contained by an appropriate amount to properly control the amount of ferrite, bainite, and retained austenite in the steel sheet microstructure, the grain sizes thereof, C concentration in the retained austenite, and the size and the number of precipitates, to thereby attain axial collapse deformation without suffering the crumbling and cracking described above. However, those disclosures may have difficulty in stably attaining axial collapse deformation without suffering crumbling and cracking, particularly in a steel sheet having a tensile strength of 980 MPa or above, and stable energy absorption to be attained through axial collapse deformation is limitedly ensured only when the steel sheet has a combination of the aforementioned chemical composition and microstructure, and thus there has been a demand for a member formed of a steel sheet with TS 980 MPa or above that is capable of suppressing fracture and breaking upon high-speed crush, so as to be stably buckled into a bellows shape.
According to Y. Okitsu and N. Tsuji; Proceedings of the 2nd International Symposium on Steel Science (ISSS 2009), pp. 253-256, Oct. 21-24, 2009, Kyoto, Japan: The Iron and Steel Institute of Japan, the member is formed of a steel sheet improved in n-value serving as a measure of the strain hardenability, so as to be formed as a collision energy absorbing member which crushes into a bellows shape in the axial direction upon collision. However, we found that even when a steel sheet having an n-value higher than 0.205 is used to fabricate the collision (impact) energy absorbing member and the member is impact deformed in the axial direction, the member may still fail to be stably buckled (crushed) into a bellows shape in some cases.
Therefore, there is a need to provide a vehicle collision energy absorbing member formed of a high strength thin steel sheet having a tensile strength TS of 980 MPa or above, which is also excellent in axial collision energy absorbing performance upon collision; and a manufacturing method therefor. When a member is “excellent in axial collision energy absorbing performance upon collision”, it means that the member is stably buckled in the axial direction and crush-deformed into a bellows shape upon vehicle collision, to thereby efficiently absorb energy of the collision, which may also be referred to as being “excellent in axial collapse stability”.